1. Field of the Invention
This invention relates to a method for analyzing a sample of a host liquid held in a vessel for the presence of an undesired liquid, and, if necessary, removing sufficient of the liquid in the vessel to remove the undesired liquid from that vessel.
2. Description of the Prior Art
Although, for sake of clarity and brevity, this invention is described in terms of an ethylene polymerization plant that produces a polyethylene product, this invention is broader in its application than polyethylene production plants.
Ethylene is polymerized to polyethylene homopolymers and co-polymers by a number of different processes to make different polymeric products such as low density polyethylene, high density polyethylene, and linear low density polyethylene which exhibits favorable characteristics found in both low density and high density polyethylenes. For sake of example only, this invention is described herein primarily in terms of a solution phase (solution) polymerization process for making high density polyethylene (HDPE).
Although originally used to make HDPE, the solution polymerization process has been adapted to copolymerization and the making of linear low density polyethylene. The system operates at lower pressures, from about 2000 to about 3000 psig, rather than the earlier high pressure, 50,000 psig, bulk or high-pressure polymerization process, and runs at temperatures of from about 300 to about 550 degrees Fahrenheit (F) using conventional Ziegler-Natta catalyst systems.
In the solution process a hydrocarbon such as n-hexane or cyclohexane, for example, is employed as the host medium (solvent) in which the polymerization reaction takes place. This host liquid solvent keeps the ethylene monomer, catalyst, and the polyethylene product in a fluid state, and also keeps these materials in intimate contact with one another to facilitate the polymerization reaction. The solvent also absorbs much of the exothermic heat from the polymerization reaction, and helps control the rate of ethylene consumption in the reaction.
The ethylene resides in the polymerization reactor for a few minutes, and, as it polymerizes, remains dissolved in the host solvent.
Downstream, the polyethylene is processed for the removal of catalyst followed by solvent separation and recovery for re-use in the reactor. The remaining molten polyethylene is fed to a drying, extruding, and pelletizing system wherein it is converted to solid polyethylene pellets. The pellets are then packaged and marketed as a product of the polyethylene production plant.
The solvent is thoroughly dried of its water content because water, being polar, acts as a poison to the Ziegler-Natta catalyst system. Only a trace of water in the solvent can adversely affect the catalyst. These catalyst systems employ, for example, titanium tetrachloride/trialkyl aluminum or other transition metals such as zirconium and vanadium in place of the titanium. These catalyst systems are well known in the art and more detail is not necessary to inform one skilled in the art.
Heretofore, to determine if water was present in a tank holding a supply solvent, a sample was hand drained by an operator into a clear container, and then visually inspected for the presence of water that naturally separates out from the hydrocarbonaceous solvent. This sample was then passed to a sump that was exposed to the ambient atmosphere. If water was observed by the operator to be present in the sample, solvent was drained from the tank into the atmospherically exposed sump for a period of time. Then another sample was taken by hand and visually inspected for the presence of water. These steps were repeated until no water was seen in the hand sample taken by the operator. This process could require the taking of a number of samples before all the undesired water was removed from the solvent holding tank.
This hand sampling process sometimes led to the removal to the open air sump of more solvent than necessary to remove the undesired water because the operator could drain solvent that did not contain water into the sump before he took his next hand sample. In other words, the operator had no effective, much less accurate, way of determining whether all the water had been removed except by trial and error hand sampling, and this could cause the removal of solvent that was not contaminated with water. This sampling process could, therefore, cause a lot of the solvent, a volatile hydrocarbon material, to be passed into the sump that was exposed to the ambient atmosphere, thereby allowing the solvent to vaporize and enter the atmosphere. Thus, a laborious hand sampling procedure was heretofore employed that not only could waste dry solvent, but delivered to the ambient atmosphere more solvent than necessary or desirable.
Accordingly, it is desirable to be able accurately to detect the presence of even minute amounts of water in as small a volume of solvent sample as possible before that solvent is employed in the polymerization process. It is equally desirable to be able to take a minimum number of samples of solvent, each sample in a minimum volume, during the sampling of that host liquid.
It is just as desirable that any additional solvent that is drained to remove water from the holding tank, is taken from the tank in a manner such that only the minimum amount of solvent is taken that is necessary to remove the water, and that minimum amount of solvent is disposed of in an environmentally friendly way.
This invention accomplishes all of the foregoing.